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Hydraulic chucks are used on industrial cnc machines. The whole point is that the opening and closing of the chuck can be controlled electronically. The form of power that actually moves the jaws is hydraulic oil.

Hydraulic chucks are included here, not because the amateur might use them, but it might be useful to him to understand why he might be better off without one.

On looking at a hydraulic chuck the first thing to notice is that there is no facility for opening or closing the jaws of the chuck. This is to be expected. What is more surprising it that there are no signs of connections to any source of hydraulic power.

fig the front of a hydraulic chuck

fig the back of a hydraulic chuck

The reason for this is that the power going to the chuck is a mechanical force coming from a tube fitted to the back of the chuck. The thread for the end of this tube can be seen in the photo of the back of the chuck.

This tube is fitted with a hydraulically driven piston fitted on the left hand side of the head stock.

When an hydraulic chuck is seen for sale on ebay all that is seen is the chuck. The hydraulic bit is never included. Needless to say, though the chuck might be expensive the hydraulic be will cost several times more.

In general, a hydraulically driven chuck only has very little movement for the jaws, just a few millimetres. This might seem to be a limitation but it is got round by the jaws being made of two parts. There is the part that is driven, commonly referred to as the master jaw and a second part that is fitted to the master jaw by means of T-nuts and bolts, commonly called the top jaw.

fig. The master jaw

Since the master jaw is always nearly in the same position it is easy to make a chuck where it is very hard for swarf to get inside the chuck.

fig. top jaw with T-nut

On a large cnc machine the usual approach to holding a workpiece concentrically is by using soft jaws. Using soft jaws the jaws get used up so it is very convenient to be able to replace the top of the jaw with new soft jaws when it has been used up.

Using soft jaws is covered elsewhere.